Most people think of the postscript as a place to tuck cute or mushy messages at the end of a note to a loved one. In reality, postscripts can be a surprisingly powerful tool for marketers.

I include a postscript (a sentence or two preceded by "P.S.") in nearly every marketing letter or email message I write for my clients. Interestingly, nine out of ten new clients will request the postscript's removal the first time they encounter one. "It's silly to add a P.S." some suggest. "This information belongs in the body of the letter," others insist. "Nobody's going to read that," still others claim.

But they're all wrong. A thoughtfully written postscript adds powerful impact to a letter and can dramatically improve its success rate. Why? Because of how people actually read letters.

You see, a common misconception is that people read letters in a linear fashion -- starting at the top and working their way to the bottom, line by line. Decades of direct marketing research proves that isn't really what happens. The vast majority of people first glance at the salutation to see who the letter is intended for (even though the envelope was presumably addressed to the reader), so the first thing your recipient sees is "Dear Bob," "Dear Mr. Smith," "Dear Customer," or whatever you've chosen. (That makes the choice more important than you may realize, by the way. The more personal the salutation, the greater the reader's immediate interest.)

The second place people look is the bottom of the letter, to see who it's from. If there is a postscript, that's where their eyes travel next. Most will read the entire postscript before deciding whether the letter is worth their time. If the message in the postscript is engaging, compelling, or intriguing, they're more likely to go back and read the entire letter -- and when they finish, they'll read the postscript again.

In other words, the postscript provides the first impression of what you're trying to sell, promote, or communicate, as well as the last impression the reader will have of your message. That's why it's generally a good idea to include one, and why you should approach it thoughtfully instead of treating it like a throwaway line.

How should you use a postscript? There are several ways that are especially effective. First, you can restate or re-present your offer or main message, as in "P.S. Remember -- we're offering our top-of-the-line veeblefetzer for 40 percent off through June 1." You can use the postscript to emphasize a key benefit of what you're offering, such as "P.S. Our veeblefetzers make you more productive by coring radishes in half the time of our competitors' models."

If your offer includes something that eliminates or reduces risk, use the postscript to say that, as in "P.S. You can put our veeblefetzer to the test for 30 days at no cost" or "P.S. We offer the only veeblefetzer with a two-year parts and labor warranty!" You can also use a postscript to share a positive testimonial, such as "P.S. Bob Smith of Veggiworks credits our veeblefetzer with increasing his radish production by 62 percent last year!" Another common tactic is suggesting that failing to act is foolish: "P.S. I don't know why anyone would pass up this offer, because there's absolutely no risk or obligation."

Pay attention to the marketing letters and emails you receive over the next month, and see how many of the most effective and compelling ones include that trusty tool. Start incorporating them in to your own correspondence, and see what effect they have.

Still not convinced? If you regularly send some kind of sales letters, perform what's known as a "split test." Divide your mailing list in half. Send one group the letter with the postscript, and the other group the letter without it. Then compare the results.